1. Field of the Invention
The present invention relates to a solid-state image sensing device, and more specifically to an improvement of a casing for housing a solid-state image sensing element.
2. Description of the Prior Art
Recently, the solid-state image sensing device is used widely in various fields. Conventionally, as shown in FIG. 1, a solid-state image sensing element (charge coupled element) 3 is mounted in a casing 2 formed with a hollow portion 1, and an opening surface 7 of the hollow portion 1 is covered with a light transmissive cap 6 formed by a light transmissive plate both surfaces of which are covered with non-reflective films 4a and 4b.
In more detail, the casing 2 formed of ceramic, for instance is formed with the hollow portion 1. In the side wall of the hollow portion 1 of the casing 2, a bottom portion 8, a first stepped portion 9, a second stepped portion 12, and a third stepped portion 13 are formed stepwise. The solid-state image sensing element 3 is mounted on the bottom portion 8. Further, an electrode of the solid-state image sensing element is connected to the first stepped portion 9 formed in the casing 2 via a metallic fine wire 10. That is, a conductive pattern (not shown) is formed in the first stepped portion 9 and the bottom portion 8 and then connected to an external electronic apparatus. Further, in the case where the fine wire 10 is grounded, the conductive pattern is not formed at the bottom portion 8 of the casing 2.
Further, a light shading plate 11 is mounted at the second stepped portion 12, and the light transmissive cap 6 is fixed to the casing 2 by applying an adhesive agent 14 onto the third stepped portion 13.
The non-reflective film 4a formed on the outer surface of the light transmissive plate 5 serves to prevent light allowed to be incident thereupon from being reflected and further to control the light entering the solid-state image sensing element 3. On the other hand, the non-reflective film 4b formed on the inner surface of the light transmissive plate 5 serves to prevent light from being reflected irregularly from the surface of the solid-state image sensing element 3 and the metallic fine wire 10 so that unnecessary light is prevented from entering the solidstate image sensing element 3.
However, the inventor of the present invention has found that there exists a problem in that charge is induced in the sensing element 3 by radioactive rays (.beta. rays) emitted from the non-reflected film, in particular the nonreflective film 4b formed on the inner surface of the light transmissive plate 5, in spite of the fact that no light is allowed to be incident upon the image sensing element 3, with the result that the characteristics of the image sensing element 3 is deteriorated (referred to as a white defect, hereinafter) and thereby the reliability of the image sensing device is degraded.
The inventor has further found that the generation of the white defect depends on the quantity of potassium-based oxide which is a material of the light transmissive plate 5.
These correlations are quite different from the conventional theory with respect to the mechanism of white defect generation due to crystal defects produced in the semiconductor wafer. In the semiconductor devices such as EPROM, although software error caused by radioactive rays (.alpha. rays, in particular) is well known, however, the correlation between the solid-state image sensing element and the radioactive rays (.beta. rays, in particular) has not been so far reported.